1. Field of the Invention
This invention relates generally to integrated semiconductor devices and more particularly to lateral bipolar transistors and a method of manufacturing such transistors.
2. Related Art
U.S. Pat. No. 4,180,827 to Harlan R. Gates and assigned to the assignee of the present invention, describes a double diffused, lateral PNP bipolar transistor having an emitter 62, a base 36 and a collector 44. In such transistor designs the emitter and base have a rectangular cross-section. The inventors have discovered that this rectangular geometric relationship for the emitter and base of a lateral PNP transistor results in certain adverse effects on the performance and breakdown voltages of the transistor. The emitter and base in such a device are self-aligned since they are both defined by the same mask shape. The base width in such a device is established by the lateral diffusion of the emitter and the base. In emitter and base regions having a rectangular cross-section, the dopant distribution is not uniform over the entire region. Dopant concentration tends to be different at the corners when compared to the sides of the rectangle and in the case of non square rectangles, dopant concentration tends to be different on the short and long sides of the rectangle. Such differences in dopant concentration produce a transistor having a variable base width thereby creating corresponding undesirable variations in breakdown voltages and other transistor characteristics. Exacerbating this problem is the fact that regions having a rectangular cross-section tend to have rounded corners of uncontrolled radius as a result of diffraction and certain other process variables.
The inventors have further recognized that there are inherent conflicting performance consequences that result from the collector doping level selected for a double diffused PNP bipolar transistor of the type described in U.S. Pat. No. 4,180,827. A relatively low level of collector doping has the beneficial effects of providing adequate punch-through and Early voltage performance and a low base/collector Junction capacitance. A relatively high level of collector doping has the beneficial effect of providing a low collector series resistance. Thus the selection of any particular collector doping level will compromise one or both of these sets of parameters.
In addition, the inventors have recognized a need for a lateral PNP bipolar transistor whose voltage and current capacities and frequency response may be easily tailored to desired levels.